Binary dithered oversampling analog-to-digital converters
Date
2009
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
In today’s world, analog-to-digital converters (ADC) play a major role. Our
modern society requires and depends on electronic devices that process the analog
input data in the digital domain, such as cellphones, audio and video systems, and
even domestic appliances. Therefore, the development of fast and accurate ADCs
have become a key issue for the industry. In general, a good ADC is the one
that achieves high resolution (low quantization noise) with low complexity. One
of the most popular techniques to decrease the quantization noise is to digitalize
the incoming signal with a sampling frequency many times higher than twice the
signal bandwith. This technique is called oversampling. Probably the most popular
oversampling converters are the Sigma-Delta (SD) modulators which use a very high
sampling frequency and a binary quantizer (for first order SD). In SD, the quantizer
is embedded into a feedback loop in such a way that the quantization noise is not
only spread over the spectrum (because of oversampling), but it is also shaped to
the upper frequencies (this is called noise shaping). The big problem with SD is that
the quantization noise spectrum presents undesired harmonics caused by the nonlinear
nature of the quantizer. To avoid this, the solution is to add an independent
signal before the quantization stage called dither. It was proven by Lipshitz and
Vanderkooy in [1] that dithering a first order SD modulator is ineffective as it turns
the modulator in constant overload. In addition to this problem, because of the
feedback loop, higher order SD modulators can be unstable. That being said, the
aim of this work is to present a simple oversampling ADC without feedback capable
of generating the minimum uncorrelated quantization noise that yields the maximumpossible SNR at the output. For that purpose, this work develops the statistical
characteristics of the optimum dither that achieves the mentioned goal for different
types of input signals.